This invention relates to a light emitting and receiving device constituted by semiconductors capable of performing both conversion from an electric signal to an optical signal and from an optical signal to an electric signal.
A light emitting diode has been generally used in a wide circle as a semiconductor light emitting element for the conversion of an electric signal into an optical signal.
Similarly a photodiode or phototransistor has been generally used in a wide circle as a semiconductor light receiving element for the conversion of an optical signal into an electric signal.
Integration of the light emitting element and the light receiving element into a semiconductor chip enables the elimination of conventional problems as described below.
For a bidirectional communication system with a single optical fiber, each of the transmitting-receiving devices is necessary to provide a light emitting element for feeding an optical signal to the optical fiber and a light receiving element for receiving the optical signal transmitted in the optical fiber. Since the light emitting element and the light receiving element are respectively independent devices, conventionally the both elements have been almost impossible to be optically coupled in good efficiency directly to an end face of an optical fiber due to the area of the end face of the optical fiber and the dimensions of the both elements. Therefore, practically the end part of an optical fiber is divided through a divider/coupler into two optical fibers at the light emitting side and the light receiving side to couple both the light emitting element to the end face of the optical fiber at the light emitting side and the light receiving element to the end face of the optical fiber at the light receiving side. However, in this arrangement, a large amount of propagation loss is caused by the divider/coupler interposed in an optical transmission passage. The above loss comprises not only the connection loss of the divider/coupler but those essentially inevitable caused by dividing light in the main line optical fiber into two optical fibers.
To eliminate the above described problem, the following method has been usually proposed. Both constructional elements of similar P-N junction, can perform conversions from electricity to light and from light to electricity. This enables the constitution of a semidouble optical communication system by coupling a light emitting diode or photodiode to the end face of a single optical fiber and allowing this element of diode to function as the light emitting element at the time of transmission and simultaneously as the light receiving element at the time of reception. However, devices designed as a light emitting diode and designed as a photodiode, even though they are the elements of the same P-N junction construction, are largely different from each other in their optimum element constitution. Thus, it is almost impossible to obtain a single P-N junction element capable of satisfying both required luminous efficiency and light receiving sensitivity. Accordingly, using a light emitting diode also for light reception, its light receiving sensitivity remarkably and must be complemented by electrical amplification in a rear stage, resulting in a system with lower noise resistance.
The above described problem in the past can be also solved by use of an optical fiber reflection photoelectric switch. The optical fiber reflection photoelectric switch is of such constitution that a light emitting element and light receiving element are coupled to the base end side of an optical fiber to irradiate light from the light emitting element to the outside from a point end of the optical fiber, then its reflected light is again guided from the point end of the above described optical fiber and detected by the above described light receiving element.